In the periodical "IEEE Transactions on Instrumentation and Measurement", Dec. 1987, pp. 873 to 878, an arrangement for processing sensor signals furnished by a capacitive sensor is described which includes a measuring capacitance influenceable by a physical measured quantity to be determined and at least one reference capacitor having a reference capacitance influenceable in another manner and furnishing a measuring effect dependent on said measuring capacitance and said reference capacitance, comprising a signal processing circuit which operates by the principle of quantized charge transport by means of switch-capacitor structures and charge balancing at least in time average and which receives said sensor signals representing said measuring effect and furnishes an output signal corresponding to said measuring effect.
Signal processing by the quantized charge transport principle by means of switch-capacitor structures and charge balancing has the advantages that offset voltages of the operational amplifiers used have no influence on the output signal, that the circuits are completely integrable in CMOS technology and that no accurate long-term reference voltage is necessary. Furthermore, from the clock pulses with which the switch-capacitor structures are controlled for performing the charge transports two clock signals can be derived, the frequency or pulse ratio of which is directly proportional to the measuring effect so that the measurement result is available in digital form.
In this pre-described circuit, one electrode of each of the capacitors of the sensor or an electrode common to them is connected by the signal line leading to the signal processing circuit to an input of an operational amplifier, the other input of which lies at a fixed reference potential, for example ground potential. By periodically actuated switches the voltages which are necessary for reverse charging in order to produce the charge packets to be transported, are applied to the other electrodes of the sensor capacitors.
A disadvantage of this pre-described arrangement is that all said electrodes are active. The function can, therefore, be disturbed if charges are coupled onto the electrodes connected to the signal line by external electric fields. Shieldings necessary to suppress such charge couplings restrict the construction and use possibilities and consequently cannot always be applied.
An object of the invention is to provide an arrangement which whilst retaining the advantages of the signal processing by the principle of quantized charge transport by means of switch-capacitor structures and charge balancing permits a one-sided fixed connection of said capacitors to any desired potential.